Hydrogen peroxide is a useful basic chemical indispensable to the food, medicine, pulp, textile and semiconductor industries Hitherto, hydrogen peroxide has been mass-produced by a continuous synthesis process in which a 2-alkylanthraquinol is caused to autoxidize to obtain the target compound, and the anthraquinone that is simultaneously obtained is reduced with hydrogen to the original anthraquinone derivative However, there is a growing need for an on-site hydrogen peroxide production apparatus. This is because a troublesome operation, e.g., repeated rectification, is necessary for purifying the mass-produced reaction product, and because hydrogen peroxide is an unstable substance incapable of longterm storage. Also, care must be taken to ensure safety in transportation and to avoid pollution.
In power plants and factories where seawater is utilized as cooling water, a technique for preventing the attachment of organisms to the inside of a condenser has been employed which comprises directly electrolyzing seawater to generate hypochlorous acid and utilizing the acid to inhibit organism attachment. However, restrictions are being placed on the use of hypochlorous acid from the standpoint of environmental conservation. This is intended to prevent hypochlorous acid from reacting with marine organisms and organic substances present in the seawater to form organochlorine compounds, which reaction products may cause secondary pollution. On the other hand, it has been reported that addition of a minute amount of hydrogen peroxide to the cooling water effectively prevents the attachment of organisms. It has further been reported that addition of hydrogen peroxide is also effective in maintaining the quality of water for use in fish breeding farms. However, there are still problems concerning safety in hydrogen peroxide transportation and pollution abatement as stated above.
Processes for producing hydrogen peroxide through the reduction reaction of oxygen gas have hitherto been proposed. U.S. Pat. No. 3,693,749 discloses several apparatuses for the electrolytic production of hydrogen peroxide, while U.S. Pat. No. 4,384,931 discloses a process for producing an alkaline hydrogen peroxide solution with an ion-exchange membrane. U.S. Pat. No. 3,969,201 proposes a hydrogen peroxide production apparatus having a carbon cathode of a three-dimensional structure and an ion-exchange membrane. However, in these processes, the amount of an alkali which inevitably generates simultaneously with hydrogen peroxide increases almost in proportion to the amount of hydrogen peroxide that is produced. Consequently, the hydrogen peroxide solution that is obtained has limited uses because the alkali concentration thereof is too high relative to the concentration of hydrogen peroxide.
U.S. Pat. Nos. 4,406,758, 4,891,107, and 4,457,953 disclose processes for hydrogen peroxide production in which a porous diaphragm and a hydrophobic carbon cathode are used to obtain an alkaline aqueous hydrogen peroxide solution having a small alkali proportion (a low sodium hydroxide/hydrogen peroxide ratio by weight). These processes, however, have drawbacks in that the control of operation conditions is troublesome. This is because the amount of electrolyte solution moving from the anode chamber to the cathode chamber and the rate of movement are difficult to control, and especially because hydrogen peroxide does not generate in a constant proportion.
In the Journal of Electrochemical Society, Vol. 130, pp. 1117-(1983), a method is proposed for stably obtaining an acidic hydrogen peroxide solution in which a cation- and anion-exchange membrane is used and sulfuric acid is fed to an intermediate chamber. Denki Kagaku, Vol.57, p.1073 (1989) reports a technique for improving performance by using united membrane electrodes as an anode Furthermore, the Journal of Applied Electrochem., 25 (1995) pp.613-627 describes electrolytic processes for hydrogen peroxide synthesis known at that time. However, these techniques are disadvantageous in cost because the electric power consumption rate is too high, and further have a drawback in that sulfuric acid is used and this unavoidably results in inclusion of the acid. Hence, a fully satisfactory process for hydrogen peroxide production has not yet been obtained.
The Journal of Applied Electrochemistry, Vol.25, pp.613-(1995) discloses various processes for electrolytically yielding hydrogen peroxide. Each of these processes is intended to efficiently yield hydrogen peroxide in an atmosphere of an aqueous alkali solution. When pure water, ultrapure water, or the like, for which an alkali such as KOH or NaOH is indispensable, is used as a feed material, the hydrogen peroxide thus produced is more valuable because it contains no impurities. The Journal of Electrochemical Society, Vol.141, pp.1174-(1994) proposes a technique of electrolysis in which pure water as a feed material and an ion-exchange membrane are used to synthesize ozone and hydrogen peroxide on the anode and the cathode, respectively. This technique, however, is impractical because the current efficiency thereof is low. Although a similar method in which the efficiency of synthesis increases with increasing voltage has been reported, this method is impractical from the standpoint of safety. Furthermore, an electrolytic process in which a palladium foil is used has been proposed. However, this process has limited uses because the hydrogen peroxide solution thus produced has a low hydrogen peroxide concentration.
In these processes for the electrolytic production of hydrogen peroxide, a two-chamber electrolytic cell, i.e., a cell partitioned into an anode chamber and a cathode chamber with an ion-exchange membrane as a diaphragm, or a three-chamber electrolytic cell, i.e., a cell partitioned into an anode chamber, an intermediate chamber, and a cathode chamber with ion-exchange membranes, is used to conduct electrolysis while feeding water to one of these electrode chambers. The electrolytic liquid feed in these processes contains an electrolyte in a concentration as low as from several 100 ppm to about 10,000 ppm so as to impart electrical conductivity.
However, the electrolytic liquid, even when containing an electrolyte, has a high resistance with an electrical conductivity of about from 100 to 10,000,000 .omega.cm. Consequently, the current density in those processes is about 5 A/dm.sup.2 at the most and is usually as low as 1 A/dm.sup.2. The prior art processes therefore have a problem in that the equipment is exceedingly large when a large amount of hydrogen peroxide is needed. In addition, the above processes have a drawback in that the consumption of electrodes is accelerated although the reason therefor is unclear. According to the experiences of the present inventors, even a platinum electrode is consumed at a rate from several to ten or more times the consumption rate in the electrolysis of ordinary electrolyte solutions.
The electrolyte is a metal salt in most cases. When an electrolytic liquid containing a metal salt is electrolyzed, the hydrogen peroxide thus produced is contaminated with metal ions. Use of this hydrogen peroxide, e.g., for cleaning semiconductors is problematic in that the metal ions contained in the hydrogen peroxide adhere as an impurity to the semiconductor surface, leading to insulation failure. Although use of ammonium salts is less apt to cause such a problem as opposed to metal salts, the ammonium ions may remain in the hydrogen peroxide thus produced to cause slight fouling.
In the case where a neutral diaphragm is used as a partition for separating an anode chamber from a cathode chamber, the two electrodes are arranged close to each other respectively on both sides of the diaphragm in order to attain a reduced electrolytic voltage. However, even when such an arrangement is employed, various electrolysis products which have been generated in each chamber move to the opposite electrode chamber. That is because of the high gas and liquid permeability of the diaphragm which again causes oxidation or reduction, thereby resulting in reduced efficiency. Since the electrolytic liquid generally has a low concentration, it has a high electrical resistance. Specifically, there are cases where the electrolytic voltage at an electrode-to-electrode distance of about 1 mm is as high as 10 V or above even when the current density is as extremely low as 1 A/dm.sup.2. Although this drawback can be alleviated to some degree by increasing the electrode-to-electrode distance, not only complete elimination thereof is impossible but the increased resistance resulting from the increased electrode-to-electrode distance results in a significant increase in power consumption. There is another problem in that the resistance loss causes considerable heat generation and this necessitates cooling of the electrolytic liquid, resulting in a further increase in power consumption.